Dear Optimist, (lovely name by the way - such a long time until March, much better to be optimistic than worried).
Your question is difficult to answer for both Ed'smum & me as this is our first experience 0f 11+. However, I can give you my impression....
I understand (and I may be wrong) that the Moray House papers contained 100 questions and many more question types than the NFER - possibly as many as 34 v 15. Obviously this need not affect the level of difficulty but it does make preparation a great deal easier! The practice papers used for the Moray House (Bond & Athey) seem to be of a similar level to the NFER / IPS used for this year's preparation; my daughter scored about the same on both types. I suppose it's fairly irrelevant anyway as the same number of children will 'pass'?? Having said that, if a test is 'easier' then the more accurate (rather than more 'intelligent') children will come out on top???
I am not in your area, so don't have knowledge of your local prep schools but I can say that - in my opinion - prep school preparation for 11+ is not all it's cracked up to be. 1 hour per week wasted in the case of many children and certainly not worth paying prep school fees purely for 11+. Of course people have many other reasons for sending their children to prep schools. Just to warn that I know of parents who have moved their children (to the children's distress) in an attempt to get them in to grammars. And wasted their money.
Good to see another Warwickshire parent, from reading your post I imagine your children will be sitting the exam in a few years time?